1. Field of the Invention
The present invention relates generally to a zoom lens optical system that is suitable for a mobile phone camera or a digital camera using an image pickup device and, more particularly, to a small physical size zoom lens optical system capable of a high degree of magnification variation.
2. Description of Related Art
Zoom lens optical systems are used in electronic digital cameras using an image pickup device. Such optical systems focus light on an image pickup device such as a Charge-Coupled Device (CCD) or a Complementary Metal-Oxide Semiconductor (CMOS). These optical systems also may include folding optics to bend an optical path and thereby satisfy packaging requirements imposed by the camera configuration. Typical folding optics include a prism or a folding mirror.
The modern cellphone contains a very efficient camera capable of extremely high quality pictures. Image quality is of the order of 8 megapixels and is achieved by the combination of extremely small pixel dimensions (typically 1.5 micrometers linearly) and a complex diffraction-limited lens. The required lens quality is that of good modulation transfer response out to at least 300 cycles/mm. This implies that lens speeds have to be faster than f/3. F/2.4 is a typical lens speed.
Many such cellphone camera lenses are of short focal length (relative to the image diagonal) resulting in total fields of view of around 65 degrees. This permits a wide variety of general photographs and allows a certain amount of digital zooming which is subject to an immediate and progressive loss of image quality. A 3× digital zoom on an 8 megapixel base system will yield only 0.9 megapixels in the zoomed image.
Such cellphones could benefit from the use of an optical zooming prime lens which is difficult to implement. The zoom lens should firstly satisfy the optical image quality parameters disclosed above which is essentially diffraction-limited with a speed at all focal lengths faster than f/3. The lens also requires a physical size compatible with the size and form factor of the modern cellphone.
It is highly desirable for the zoom lens to be incorporated within the body of a modern cellphone, which implies a (possibly folded) design that has elements comparable in size, and certainly not significantly larger than, the thickness of a typical cellphone, for example on the order of about 7 mm.
As one considers the current state of the art in small camera zoom lenses, it is clear that the widely used design forms will not satisfy the criteria outlined above. Modern small camera zooms invariably use design forms that are simplified by allowing the long focal length end to become significantly slower than the speed at the wide end. This is facilitated by both allowing the aperture stop to move axially and to sometimes change diameter as the lens is zoomed. Lens f/numbers typically change from f/3.5 to f/5.6 as zooming proceeds. This presents a significant opto-mechanical challenge for a cellphone zoom. But, far more importantly, this slower f/number at the long end of the zoom is impermissible for a cellphone with extremely small pixel elements. The required image quality will be unattainable because of diffraction limitations.
Therefore, it is highly desirable to have a zoom lens design that both satisfies the physical (small dimension limitation) and the optical (near diffraction-limited performance at spatial frequencies compatible with very small pixels) requirements of the modern cellphone. It is also highly desirable to have a zoom lens that will permit the zoom lens to fit entirely within the cellphone body and not protrude while zooming. It is also highly desirable to have a zoom lens that is physically robust and able to withstand extremely stringent drop force requirements required by modern cellphones.